Harmonic drives have been used as motors and actuators in many electro-mechanical applications. One type of harmonic motor has a rotatable rotor and a surrounding non-rotatable stator. The rotor makes a single point of contact with the inner circumference of the stator. The single point of contact rotates around (i.e. rolls around) the inner circumference of the stator. The rotor rotates a few degrees about its longitudinal axis for each complete rotation of the single point of contact about the inner circumference of the stator. In one modification, the outer circumference of the rotor and the inner circumference of the stator have gear teeth. Such motors find use in high torque, low speed motor applications.
In one known variation, the rotatable rotor is above a non-rotatable stator, the rotatable rotor flexes or wobbles downward to make a single point of contact with the stator, the single point of contact rotates around an “inner circumference” of the stator, and the rotor rotates a few degrees about its longitudinal axis for each complete rotation of the single point of contact.
In another type of harmonic motor, a shaft is surrounded by a shaft driving member which is brought into a single point of contact with the shaft by electro-restrictive devices, wherein the rotor rotates a few degrees for each complete rotation of the single point of contact around the inner circumference of the shaft driving member.
Harmonic drive gear trains are known. In one known design, a motor rotates a “wave generator” which is an egg-shaped member, which flexes diametrically opposite portions of the surrounding flex-spline gear, which is inside an inner gear. As the diametrically opposite teeth of the flex-spline gear contact the teeth of the outer gear, the rotatable one of the gears rotates with respect to the non-rotatable one of the gears.
U.S. Pat. No. 6,664,711 to T. Baudendistel describes a harmonic motor which includes a first annular member, a second member, and a device for flexing the first annular member. One of the members is rotatable about the motor's longitudinal axis, and the other member is non-rotatable. The flexing device flexes the first annual member into at least two spaced-apart points of contact with the second member, and sequentially flexes the first annular member to rotate the at least two spaced-apart points of contact about the longitudinal axis which rotates the rotatable one of the members about the longitudinal axis.
By using at least two points of contact between the members, the rotatable one (i.e., the rotor) is being driven by at least two points of contact by the non-rotatable one (i.e. the stator or rotor driving member). Driving the motor with at least two points of contact provides a more robust and more smoothly operating motor than is otherwise provided by the prior art.
In certain applications, linear actuators are preferred to motors. For example, a brake system for a motor vehicle, and in particular an automotive vehicle, functionally reduces the speed of the vehicle or maintains the vehicle in a rest position. Various types of brake systems are commonly used in automotive vehicles, including hydraulic, anti-lock or “ABS”, and electric or “brake by wire”. For example, in a hydraulic brake system, the hydraulic fluid transfers energy from a brake pedal to a brake pad for slowing down or stopping rotation of the wheel of the vehicle. Electronic systems control the hydraulic fluid in the hydraulic brake system. In the electric brake system, the hydraulic fluid is eliminated. Instead, the application and release of the brake pad is controlled by an electric caliper.
Generally, the electric caliper includes a motor and a gear system. Typically, either a few large gears or many small gears for the gear system are needed to achieve the necessary load transfer. Also, the geometry of the motor influences its efficiency, since the preferred shape is long and thin. However, there is a limited amount of space available in the wheel for packaging the type of gears and motor necessary to obtain the same load transfer as in the hydraulic brake system. Therefore, space limitations constrain the use of an electric caliper in an automotive vehicle.
U.S. Pat. No. 6,626,270 to D. Drenner et al. describes a brake caliper which includes an electric motor having a shaft and an associated gear system including first and second planetary gears rotatable engaged with the motor shaft. At least one of the planetary gears is engaged with the shaft and a piston, and is operatively engaged with a first carrier. The other planetary gear is operatively engaged with the first stage carrier and a second carrier. A ball screw is engaged with the second stage carrier for rotation therewith, and a ball screw nut is operatively engaged with the ball screw.
Although having many advantages to mechanical brake systems, more recent prior art systems based upon hydraulic pressure behind a piston or, alternatively, an electric motor employed to turn a ballscrew to move a piston to create clamping force in a brake caliper also have drawbacks. Hydraulic brake systems employ a closed hydraulic system filled with hydraulic fluid to control the piston. This approach, although currently common in the industry, can present adverse environmental, assembly, control and safety aspects. Likewise, the electro-mechanical system approach employs multiple parts, which have certain inefficiencies, namely a motor, planetary gear set and ballscrew. These components, in addition to being expensive and difficult to assemble and maintain, also can have the disadvantage of high inertia and back-drivability resistance.
It is, therefore, a primary object of the present invention to provide an improved harmonic drive configured as a linear actuator suitable for automotive brake caliper applications in brake by wire systems, which overcomes known shortfalls of existing devices without adding to part count, manufacturing complexity, cost or reduced robustness.